This application pertains to the art of plastic extrusion and more particularly to the art of simultaneously extruding a plurality of plastic materials in layers. The present invention finds particular application in the extrusion of layered plastic cylindrical sleeves for manufacture into corrugated tubing products. It is to be appreciated, however, that the invention has broader applications including manufacturing smooth walled circular pipe or tubing products, hollow rounded rod or support structures, rounded and nonrounded tubular products, and the like.
Heretofore, extruders for corrugated and other tubing products have most common extruded a single component, nonlayered sleeve of plastic material for corrugating or shaping into the tubing product. Commonly, a single component extruding die had a single, axial inlet. A conical divider divided the received flow of plastic into a cylindrical form. The cylindrical plastic flow was divided among a circle of apertures disposed in the spider. On the other side of the spider, the plastic from the spider apertures merged together into an outlet cylindrical passage. Various circumferentially uniform, annular restrictions and circumferentially enlarged annular areas or plenums were disposed in the outlet cylindrical passage to encourage the plastic to flow together into a smooth uniform sleeve. The spider provided mechanical support for the structures which defined the cylindrical passages on its upstream and downstream sides. Particularly, it supported the structures which defined the inner surfaces of the cylindrical passages.
Commonly to manufacture layered plastic tubing, a feedblock was positioned between the extrusion die inlet and two or more plastic extruders, such as is shown in my earlier U.S. Pat. Nos. 4,125,585 and 4,208,175. In the feedblocks, two or more flows of plastic material where shaped into cylindrical, concentric plastic sleeves to form a layered plastic flow. The generally cylindrical layered plastic flow was received by the axial inlet of the extrusion die. Of necessity, the layered plastic flow was divided by the spider apertures and recombined in the outlet cylindrical passage. To make additional layers, additional feedblocks were positioned in series upstream from the extrusion die.
One of the problems encountered in the prior art is preventing the layers from intermixing, particularly as the layered flow is divided and recombined around the spider. Dividing and recombining a layered flow through the spider apertures requires that the plastic materials be rheologically compatible. If the plastic materials have significantly different flow characteristics, the thicknesses of the layers vary as the flow recombines downstream from the spider. Rheologically noncompatible plastics in the layered flow become intermixed and may even become striped with one layer protruding through another. Such intermixing not only causes a visually unattractive product, but also losses the physical properties that the layering was designed to achieve.